The Equine Eye

It is a common enough scenario. The wind is blowing as you lead your riding mount, racing, or performance horse from the stable to the paddock for exercise. A gust of wind sends a piece of paper skittering past on your right. Your horse, despite your quiet, steady hand on the lead shank, acts as though someone has shot off a cannon. It leaps to the left, smashing into you in the process. About that time, a piece of canvas in the hay shed to your left begins flapping in the breeze and your horse leaps to the right, this time nearly pulling the lead shank from your grasp.

What has gotten into this critter to be so rambunctious on this day when the route you are taking is identical to the one you've traveled with the animal day after day in the past?

It is in the eyes.

You looked at the skittering piece of paper and the flapping canvas and immediately recognized what they were and didn't give them a second thought. Your horse didn't see them the same way you did, and this set off an alarm, a part of its genetic makeup, that traces back hundreds of thousands of years.

We must first understand that in nature's food chain, the horse is a prey animal. For centuries, equines served as tasty meals for lions, tigers, and other beasts of the wild that would lie in wait before suddenly springing to the attack, or, in more rare cases, would simply seek to outrun the horse.

Nature did not turn its back on the horse and leave it defenseless against predators. Equines are blessed with keen hearing, an unusually broad field of vision, and the ability to run at speed to flee from danger.

The reason we often fail to understand why a horse acts as it does under certain circumstances--such as a windy day with objects blowing about--might stem from our lack of knowledge about the way in which the equine eye functions. We see what we see and believe that the horse sees these objects in the same way.

Not so.

As humans, we fall into the predator, rather than the prey, category. As such, we are equipped with binocular vision. Our eyes are located in the front of our head, only a short distance apart, just like other predators, such as lions, dogs, and cats. A predator's eyes are designed that way and so situated to enable it to focus on prey that is in front of it.

The human eye is a lot like an auto-focus camera, making use of disc-like lenses that are attached to powerful muscles, known as ciliary muscles. These muscles very quickly flex or relax to adjust the lens shape as needed. If we want to look at something far away, we simply stare at the object and, in a flash, the lens focuses on the object and we can study it. If the object is up close, the same thing happens, only now the focal point is much nearer to us.

If we want to look at something to the right or left, we simply turn our heads and the ciliary muscles and the lenses do the rest, sending a sharp image to the retina, which is a mass of nerve receptors on the back wall of our eyeballs. The retina then transmits the image to the brain--all in the proverbial wink of an eye.

When we have a problem with not being able to see well, either at a distance or up close, we can correct the deficiency with eyeglasses or contacts.

The horse does not have this luxury, nor does it have the same type of eye structure as we do.

Because it is a prey animal, the horse has both monocular and binocular vision. Its monocular vision is the result of having one eye located on each side of the skull instead of both eyes in the front. This means that the horse has far greater peripheral vision than we do. In fact, the horse has only two blind spots. One is directly in front of it when it is within about four feet of an object, and the other is directly behind it. This blind spot is eight to 10 feet in length and, like the front-end blind spot, is cone-shaped.

While the horse can do nothing to eliminate the blind spots when looking straight ahead, it can quickly turn them into its field of vision by moving its head from one side to the other.

When one realizes that this frontal blind spot exists in all horses, the feats of Grand Prix jumpers--and even hunters--become all the more amazing. When the horse reaches a point about four feet in front of a jump, the obstacle simply disappears from its field of vision. It has had to measure the jump and its own takeoff point on the approach, then put supreme faith in the rider who is guiding it toward the imposing barrier.

It thus comes as no surprise that jumping is not something horses do naturally. In the wild, horses normally will skirt an object rather than jump it. Only if hard-pressed during flight will they choose the option of jumping a barrier, rather than going around it.

How Horses See

As mentioned earlier, in man and most animals, visual focus is obtained by adjusting the shape of the lens, or moving the lens toward or away from the cornea, that transparent part of the coat of the eyeball that covers the iris and pupil and admits light to the interior.

In most animals, the retina is a consistent, smooth, concave surface. The horse's eye is different. Its retina is more concave in some places than others. In addition, some sections are nearer the cornea than others.

While humans and many other animals have strong ciliary muscles to facilitate lens adjustment, the horse's ciliary muscle is underdeveloped by comparison and does a poor job in helping the horse to focus.

Thus, the horse is dependent on the retina for good focus and clear vision. The horse retina is a thin, transparent membrane, lining the back wall of the eyeball. It rests upon a black surface, known as the choroid coat. The choroid is dark in color, and this enables the light rays to exert their full impression on the retina.

This means the horse can't focus its eyes as we do. Instead, it makes use of the irregular surface of the retina by lifting, lowering, or weaving its head. This is why a horse sometimes will raise and lower its head while staring at an object. It is adjusting the focal length between its eye or eyes and the object, as well as adjusting the angle of view, until the image falls into focus on a portion of its retina.

Because the horse's eyes are located on either side of its head, it is able to view two separate pictures at the same time, with each eye transmitting its own, separate messages to
the brain.

It's a wonder horses aren't in a constant state of confusion. Sort of like watching two TV sets, located side by side, at the same time and trying to absorb and understand the images presented on each.

Perhaps this is why the flight instinct resides so strongly in the horse. Seeing a set of confusing images, its instinct often is to flee first and figure out exactly what it saw later. More often than not, "flight" overrides "fight" in horses and the reason, usually, is that its brain can't instantly sort out the confusing images being presented by its eyes.

There is another unique aspect to the equine eye. The eyeballs operate in a manner similar to bifocal glasses. This makes sense when we ponder the fact that the horse is a grazing creature. When its head is lowered and it is looking through the upper portion of the eyes, it can continually survey the horizon for the approach of predators. If there is something up close that demands its attention, the horse will raise its head and examine the object through the lower portion of the eye.

It works about the same as the far-sighted person who uses the magnifying portion of bifocal eyeglass lenses to read a book, but looks over the top of the magnifiers to identify who just drove up the driveway.

However, just like the far-sighted person with bifocals, the horse can't see well at a distance through the bottom portion of the eye. If it wants to zero in on something with binocular vision, it must look through the upper portion of the eye. To facilitate this binocular approach, the horse will raise its head, prick its ears, and hold its head still and in a perpendicular position.

Let's assume we are riding across a meadow on a quiet day. The horse is picking its way along the trail in casual fashion. A deer flashes from cover on a hillside several hundred yards in front of us. Our eyes immediately focus on it as our lenses adjust, and we know in an instant that it is a deer.

The horse sees the flash of movement and immediately its head comes up, its ears prick forward, and it might even come to a full stop as it focuses its binocular vision on what had appeared as a blurred, moving image. At this point the horse is using the upper portion of its eyes as binoculars.

Once the average trail horse sees the moving object with clarity and realizes that it does not pose a threat, it immediately relaxes and continues along the trail. All instinctive worries about a fearsome predator have vanished. Normally, horses have a short span of concentration. If there is nothing that rings instinctive alarm bells, primarily as the result of rapid movement, in their field of vision, objects encountered generally don't trigger their interest or curiosity.

Rarely does a horse focus its attention on a still object in the distance. Movement stimulates its attention and interest. The horse wants to know immediately what the object is and which direction it is traveling.

Something else to ponder. A horse's eyeball is the largest orb found in any land mammal. This means it also has an oversized retina.

The result? Objects seen by the horse are magnified. To a horse, an object might seem a full 50% larger than it does to a human. Thus, a large dog barking or snapping at its heels might appear as large as a small pony.

In addition, if the dog is running to first one side of the horse, then the other, first one eye sees the canine and reports to the brain, then the other eye does likewise. Now the horse is receiving two messages about the troublesome dog and the result may well be a fractious animal that prances from side to side in an attempt to keep the troublesome creature in visual focus.

It is for this reason that horses must be trained to be two-sided. If you are working in a corral or arena and are going to the left on your horse, it is seeing activity inside the circle with its left eye. Then, when you turn to the right and begin circling, you might wonder why the horse has become tense. Seeing the activity through its right eye has sent a whole new set of signals to its brain.

Accomplished trainers of horses used for trail riding and various forms of competition make certain that the animal does not become "left-sided" or "right-sided." When teaching the animal to lead, they work from both sides and do the same when riding--schooling it as much to the right as to the left and vice versa.

For example, a horse which always has a rider mount and dismount from the left side, might become totally disconcerted if the rider mounts or dismounts from the right. Its brain would be receiving a whole new set of images. This would not be a good situation if one were forced to dismount on a precipitous mountain path and the only safe side to do so was on the horse's right.

In order to maintain its own inner comfort zone, a horse must see an object with both eyes or, at least, have processed the same image with each eye separately.

With some Thoroughbreds and other racehorses, the acute peripheral vision that horses possess can present a problem. The horse might be doing extremely well in the race until it heads down the home stretch and sees a mass of arm-waving, shouting people with its right eye, or has another horse range up on one side or the other. There are horses which become unnerved at the sight and will swerve to the rail or break stride. To solve the problem, a trainer might have to use blinkers so that the horse can't see the crowd or approaching competitors, but must focus on what is in front of it.

The same is true of many show horses which pull carts and draft horses which are hitched to wagons or other heavy equipment. Most bridles for these horses are equipped with blinkers (blinders) so the horse doesn't become frightened by the sight of a shadowy object that might be weaving in and out of its rear blind spot.

The fact that the object is close doesn't help. Generally speaking, horses are excellently equipped to see well at a distance, but are not as adept at seeing objects close up.

Thus, we have an explanation for the horse which is startled by a sudden movement just behind or beside it. It hasn't had an opportunity to focus on the object, but its remarkable peripheral vision has picked up the quick movement.

Remember the opening example involving a horse being led to a paddock on a windy day and is startled by skittering or flapping objects, even though it is traveling a familiar path? Often another of the horse's senses is involved when such a happening results in a nervous horse--hearing.

The horse has an acute sense of hearing, generally far superior to that of a human. As a result, on a windy day, its ears are picking up a myriad of sounds while its eyes are detecting all sorts of movement. It can make a horse downright nervous while sorting it all out.

Night Vision

There is one area in which the equine eye is definitely superior to the human eye--night vision. Generally speaking, a horse's night vision is a good 50% better than a human's.

This writer can clearly recall a trek into the mountains of Bridger-Teton National Forest in Wyoming. I was riding a Thoroughbred which had been rehabilitated from a track injury. The big gelding had become an excellent trail horse. For a number of reasons (that's another story), our group was overtaken by darkness with some miles to go before we would reach our base camp. The Thoroughbred was in the lead of our group of riding and pack animals.

With no moon in a cloudy sky, it was so dark that the human eye could not make out the trail. Yet, that Thoroughbred, walking along on a slack rein (time for me to simply trust the horse), led the way safely down the twisting, sometimes difficult, mountain trail without ever missing a step.

Why?

Because the equine eye contains a reflective panel on its retina. This panel pulls in what little light is available at night, enabling that horse to have far better night vision than its rider.

One aspect of a horse's vision is inferior to that of humans--the perception of colors. While a horse can perceive some colors, such as green and blue--perhaps to help it locate edible forage--it pretty much functions in a gray world.

However, that is only a minor deficiency, and might not even be a deficiency. After all, horses aren't startled by bright colors.

Recurrent Uveitis

Unfortunately, the extraordinary equine orb is not immune to injury and disease. One of the most common causes of blindness in horses is equine recurrent uveitis. This is definitely not a new kid on the block in the disease family. Uveitis was initially recognized and described in the Fourth Century A.D.

Through the years it has been given a number of names, the two most common being moon blindness and periodic ophthalmia.

Uveitis, simply stated, is inflammation of any part of the uveal tract--the iris, ciliary body, or choroid.

Anterior uveitis refers to inflammation of the iris and ciliary body. Posterior uveitis is inflammation of the choroid with frequent secondary involvement of the retina and optic nerve.

Whether the disease is localized to a single segment of the uveal tract or whether it affects multiple areas, uveitis has the potential to cause permanent damage to the eye and might result in blindness.

The onset of uveitis can be triggered by bacteria, viral infection, trauma, intraocular parasites, or a variety of other causes.

R. David Whitley, DVM, MS, Dipl. ACVO, at the College of Veterinary Medicine, Auburn University, succinctly penned a description of uveitis this way: "Equine recurrent uveitis is a severe, visually disabling disease characterized by three clinical stages: an acute phase, a subacute phase, and a chronic or quiescent phase. Periods of clinical quiescence alternate with episodes of active (acute) uveitis at varying intervals. In many cases, initially one eye is affected with recurrent episodes, then the second eye becomes involved. As the disease progresses, the episodes of active uveitis appear to become more frequent."

Unfortunately, the disease is not rare. One estimate suggests that 10-25% of the more than six million horses in North America might be suffering from uveitis.

In another study undertaken at Auburn University, this one headed by Susan A. McLaughlin, DVM, MS, a review was made of Veterinary Medical Data Base records from January 1986 through December 1990. The examination data involved 79,037 horses.

Of that number, 860 had uveitis and 437 had cataracts. A total of 145 horses were listed as having both uveitis and cataracts.

(Cataracts caused by uveitis, according to McLaughlin, are not considered good candidates for cataract surgery because of the concurrent pathologic changes in the eye and the high rate of post-operative complications, both of which result in a low surgical success rate.)

"In the population examined," reported McLaughlin, "the relative risk of a horse with uveitis developing cataracts versus the risk of a horse without uveitis developing cataracts was 42.5%."

The breed with the highest percentage rate of both uveitis and cataracts was the Appaloosa. The prevalence rate for Appaloosas with uveitis was 42 per 1,000 horses examined, with the prevalence rate for cataracts being 14 per 1,000.

One of the lowest prevalence rates recorded was for the Thoroughbred, with seven cases of uveitis per 1,000 examined, along with two cases of cataracts per 1,000.

One of the culprits suspected as being a cause of uveitis is leptospirosis, a disease caused by an infectious organism that often is found in contaminated water. In addition to uveitis, the organism might result in various other disease conditions, including fever, jaundice, infertility, abortion, blood in the urine, and death.

Ann E. Dwyer, DVM, of the Genesee Valley Equine Clinic in Scottsville, N.Y., last year reported on an in-field study seeking to determine if there was a strong relationship between leptospirosis and uveitis. She concluded that there was.

Dwyer and colleagues studied the medical records of all horses in which uveitis had been diagnosed and of all horses which were tested for leptospiral seroreactivity at the Genesee Valley Equine Clinic from Sept. 1, 1986, through Sept. 30, 1993.

Of the estimated 9,500 horses which had been examined during the study period, 130 (1.4%) had a diagnosis of uveitis.

Serum was drawn from 112 of the horses with uveitis and from 260 horses without uveitis in an effort to learn if there was a correlation between leptospirosis and uveitis.

The results of the study demonstrated that a strong link does truly exist. Of the 112 horses with uveitis 63 (56%) tested positive for leptospirosis and 49 (44%) tested negative. In contrast, of the 260 horses without uveitis, only 23 (9%) tested positive for leptospirosis and 237 (91%) tested negative.

As was the case in the Auburn study, the Appaloosa breed showed a strong tendency toward the disease. Of the 112 horses with uveitis, 28 (25%) were Appaloosas compared with 10 of the 260 (4%) horses without uveitis.

"The association between breed and uveitis was significant," reported Dwyer. "The odds of finding uveitis were 8.3 times greater in Appaloosas than in all other breeds combined."

Not only does research indicate that Appaloosas are more apt to get uveitis than other breeds, it also appears to have a more serious effect on their vision. Of the 28 Appaloosas with uveitis, eight (29%) lost vision in one eye and 11 (39%) were blind in both eyes. Of all non-Appaloosas with uveitis, 23 of 84 (27%) were blind in one eye and seven (8%) were blind in both eyes.

Dwyer summarized the study findings, in part, this way:

"Uveitis is an important disease in horses in the Genesee River Valley of Western New York; a prevalence of one percent was noted in the practice population. This prevalence represented only cases detected by direct ophthalmoscopy under field conditions, and thus may be under-reported. Vision was lost in one or both eyes in 44% of the horses with uveitis in this study. Approximately a third of the cases in this study were observed in horses 10 years of age, in agreement with reports that this disease often affects young, otherwise healthy horses.

"Uveitis was associated with seroreactivity to (leptospirosis) in 56% of the 112 cases of uveitis," she added.

"In this study, blindness in at least one eye was more prevalent in seropositive horses with uveitis than in seronegative horses with uveitis. Because both groups were treated similarly, this finding suggested that the prognosis for vision in horses with uveitis associated with leptospiral seroreactivity is poorer than that for horses with uveitis attributable to other causes.

"The greater prevalence of uveitis in Appaloosas may be related to the major histocompatibility complex, specifically to the equine lymphocyteantigen subtype in particular Appaloosa breeding lines."

The symptoms vary for uveitis, depending on where in the eye the disease is manifested. The prudent horse owner will consult professional help when any abnormality of the eye makes its presence known, because prompt and aggressive treatment of each episode of uveitis is extremely important in the maintenance of visual function.

Symptoms might include an overflow of tears down the horse's cheek, continued and rapid blinking, becoming sensitive to light, and visual impairment.

Treatments with corticosteroids and/or nonsteroidal anti-inflammatory drugs are often the weapons of choice against uveitis.

Generally, the goals of treatment are to prevent structural damage to the eye, such as cataracts or retinal atrophy, and to reduce inflammation.

There are also some specific management practices that can be instituted to facilitate the treatment process. Whitley suggested these practices in his report:

"A small amount of petroleum jelly can be spread on the skin beneath the eye to help prevent irritation by the medication and resultant dermatitis. Ocular discharge and medication remnants should be removed from the eye once or twice a day.

"Feeding hay on the ground or floor rather than in a hay rack or net will decrease the amount of particulate matter near the eye. Spraying the hay with a mist of water will aid in decreasing dust exposure. Reducing light exposure can be achieved by stall confinement or by ocular protector hoods or fly nets. The horse should be walked or allowed paddock exercise during dim light early in the morning or after sundown.

"Flies should be controlled, especially around the face, by appropriate means. Fly nets may serve a dual purpose of fly control and limiting light exposure. Wipe-on or roll-on insecticide products are preferred, since sprays may inadvertently contact the cornea or conjunctiva. While horses are receiving medication, the owner or technician should be instructed to monitor appetite, water consumption, and fecal consistency. While the horse is in the hospital, it is recommended that vital signs, digital pulses, and gut motility also be monitored at least once daily. If animals are receiving systemic non-steroidal anti-inflammatory drugs or corticosteroids bi-weekly, complete blood counts and serum creatinine levels should be considered.

"Identification and treatment of the inciting cause of the uveal inflammation are important in resolving and preventing uveitis. However, determining the cause is usually difficult."

The way in which horses respond to treatment and the prognosis for them varies from animal to animal. Some horses experience a single episode, respond favorably to treatment, and have no recurrence. Others respond poorly to treatment, sustain structural damage, and have repeated and frequent episodes that cause them to lose their vision.

There is much that is known about uveitis, but even more that is unknown. Early detection and immediate and aggressive treatment are the appropriate responses on the part of the horse owner.

There are a variety of other eye disorders that afflict the horse, ranging from malignant tumors to fungal attacks, but the most prominent as it affects eyesight is uveitis.

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